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Q.
When Cleaning PCBs, Can Regular Tap Water (City Water) Cause Contamination, and/or Should DI or Distilled Water Be Used?
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A.
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If you must use water, tap water is bad news. It carries minerals that will  re-contaminate the board. De-ionized water is the trick you need. Really high-purity D-I water will cut through steel and is a formidable cleaning agent. The purity of de-ionized water is measured in ohms of resistance (normal water is slightly conductive, which is why the life guards always close the swimming pool when thunderstorms blow in). At MicroCare, we package our D-I water to about 1 megaohm (1 million ohms) of resistance, and that works pretty well. In the semiconductor industry, they use D-I water at ten and twenty megaohms.
(In contrast, distilled water has had the particulate removed, but is insufficiently “hungry” to do a good job cleaning.) The problem, of course, with D-I water is there is no convenient way to keep it pure. As soon as you walk across the room with a jar of D-I water, it’s grabbing contamination from the air and losing its cleaning power. Years ago, MicroCare previously packaged D-I water in an aerosol can, and that was a great product because the D-I water came out “hungry” for ionics. Due to boring technical reasons, we no longer make that product. But the best success with D-I water is in closed cleaning systems, not on the benchtop.
By the way, if you are cleaning with water, try to warm it up. Warm water is better; it has a higher energy level and will do a better job dissolving inorganic (salt) residues. However, unless you use D-I water, this by itself will generally not remove organics very well – for example, water cannot remove fingerprint oils. Normally, some addition cleaning solvent or power (mechanical scrubbing) is needed to dislodge those residues. The problem with trying to use warm water is that there is no convenient or affordable way to keep the water warm at the workbench, and pretty soon people end up cleaning with cool water again, to uneven results.
Revised 8-23-2008
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